The present invention relates generally to measurement standards and more particularly to a portable resistance standard capable of adjusting its actual value to a nominal value.
The unit of measurement for resistance is an ohm. The legal ohm is defined by an agreement between national laboratories on the value of the Quantum Hall Effect Resistor (QHR), a fixed reference value which cannot be adjusted. Once the value of the ohm is established, the value is disseminated to a number of working standard resistors via Cryogenic Current Comparators (CCC""s). The working standard resistors are then compared against other resistance standards sent in for calibration by commercial, private, or other public laboratories. The cost, size and sensitivity of QHR""s and CCC""s does not permit their easy transport. Thus in practice, the legal ohm is disseminated by the transport and maintenance of passive resistance standards.
Prior art passive resistance standards are affected by external environmental and measurement system effects. For example, temperature, relative humidity, barometric pressure, self-heating and thermoelectric effects caused by an applied current, inductive effects at varying frequencies, and drift due to aging, among others, each have an effect on prior art resistance standards. To minimize some of these effects, prior art resistance standards are typically placed in a constant temperature oil or air bath. Constant temperature baths, however, are large, non-portable, and expensive to maintain and monitor.
Higher quality resistance standards are typically constructed of Manganin or Evanohm(copyright) wire, strip, or ribbon, which is heat treated to reduce the material""s coefficient of temperature. The standards are manufactured to possess low thermoelectric and inductive effects and hermetically sealed to reduce the effects of humidity and barometric pressure changes. High quality resistance standards, however, require a constant temperature bath to reduce temperature effects. Additionally, high quality resistance standards are easily damaged by excessive current, exhibit drift with age, and provide no means of testing the effects of the other components in a complete measurement system. Furthermore, the accuracy of high quality resistance standards is degraded by self heating during measurement and by measurement signals other than DC or low frequency AC signals. The working uncertainty of high quality resistance standards is also laborious to calculate and maintain.
High quality resistance standards are only capable of realizing a single resistance value; thus multiple high quality resistance standards must be used for each resistance value. For example, two high quality resistance standards must be used to generate a 0.01 ohm value and a 1 ohm value. The use of multiple high quality resistance standards increases the expense, size, and complexity of the complete measurement system.
Thus, there exists a need for a resistance standard having improved accuracy, that is more immune to external environmental and measurement system effects, that does not require the use of constant temperature baths, that can realize multiple resistance values, that integrates uncertainty calculations into its value, and that overcomes other limitations inherent in prior art resistance standards.
One aspect of the present invention relates to a method for improving the accuracy of a resistance standard comprising ascertaining baseline characteristics for a resistor element, determining the actual resistance value of the resistor element, selecting a target resistance value for the resistor element, and adjusting the actual resistance value of the resistor element to match the target resistance value of the resistor element.
Ascertaining the baseline characteristics for the resistor element may further comprise at least one of determining the resistor element""s coefficients of temperature; measuring the resistor element""s frequency response; and determining the resistor element""s drift due to age.
Additionally, determining the actual resistance value of the resistor element may further comprise determining the temperature of the resistor element, determining the frequency of an applied measurement signal, determining the age of the resistance standard, and calculating the actual resistance value of the resistor element using at least one of the measured temperature, frequency, and age, and the baseline characteristics. Adjusting the actual resistance value of the resistor element may comprise altering the temperature of the resistor element to realize the target resistance value.
Another aspect of the present invention relates to a self-adjusting resistance standard comprising a resistor element sealed within an element assembly, a temperature and frequency measuring circuit for measuring the temperature of the resistor element and the frequency of an applied measurement signal, a heating/cooling assembly for raising and lowering the temperature of the resistor element, a temperature controller for controlling the heating/cooling assembly, and a control system responsive to the measuring circuit to control the heating/cooling assembly to maintain the value of the resistor element. The control system may include a CPU and is operable to store the baseline characteristics of the resistor element, retrieve the temperature of the resistor element from the temperature/frequency measuring circuit, retrieve the frequency response of the resistor element, retrieve the frequency of an applied measurement signal, store uncertainty components, calculate expanded uncertainties, determine the actual resistance value of the resistor element, and adjust the actual resistance value of the resistor element to match a target resistance value using the temperature controller. The CPU also stores measurement system uncertainty components and calculates system uncertainty.
The present invention provides a resistance standard having improved accuracy, that is immune to external environmental and measurement system effects, that does not require the use of constant temperature baths, that contains multiple resistance values, that permits evaluation of measurement system sensitivity and accuracy, that includes uncertainty calculation, and that overcomes other limitations inherent in prior art resistance standards. Those advantages and benefits, and others, will be apparent from the Detailed Description below.